My question is similar to this - Rude to ask a book chapter. I have bought the first of two parts of a research monograph. The second part is even more expensive (around $150) and in my currency it is prohibitively expensive. I do not know the professor and he does not know me either. On the other hand it is possible we may collaborate in the future as we are in the same field. Is it appropriate to send him an email and ask him for a discount on his book? Would it harm a professional relationship later on in my career?
I don't think it is rude to ask for the preprint of the book. In fact, many authors publish the preprint for books on arxiv(I can give examples of this). However, the rudeness depends on the culture and has many dependencies. Finally, you can use Libgen for downloading/uploading books, and many authors put their book also here, although they would not admit it.